The endothelium is the innermost layer of tissues forming the cornea, consisting of a single layer of flat polygonal cells. One purpose of the endothelium is to control water content and, thus, permit suitable hydration of the cornea. Accordingly, the shape and number of cells in the endothelium influence the quality of one's vision. As the transparency of the cornea depends on a rather delicate balance of factors, there are a number of diseases that can readily disrupt this balance, cause a loss of transparency, and, thereby, hinder the quality of vision.
Endothelium cells in children and young people are typically hexagonal in shape. These cells, however, do not reproduce themselves. At birth, the density of endothelium cells is about 4000 per square millimeter but, as the years pass, the cells begin to change in shape, and the total number of cells decreases. In an adult, the average density is about 2700 cells per square millimeter, with a range of about 1600 to about 3200 cells per square millimeter. The loss of endothelium cells with age is accompanied by two main morphological changes: (i) the presence of cells with different surface areas, and (ii) an increase in the number of cells that are shaped differently from their original hexagonal shape.
Evaluation of the corneal endothelium has been found useful for providing a first clinical indication as to the potential risks of surgery, and for verifying a diagnosis or the effectiveness of a particular therapy. In these evaluations, it is considered particularly important to observe heterogeneous portions of the endothelium, such as intracellular and intercellular areas of no reflectance (dark spots), hyper reflective areas (bright spots), empty areas in the cells layer (guttae), bubbles, as well as Descemet's membrane rupture lines.
Such portions of the endothelium can be checked relative to the evolution of the various diseases of the endothelium which are of an inflammatory or dystrophic nature. Quantitative evaluation involves the assignment of a numeric parameter to a selected photographic field, which parameter is used to study variations in the endothelium over time, or for comparison between different patients.
The most readily accessible parameter is the average cellular density, obtained for comparison purposes by counting the number of cellular elements. A first evaluation method, in this regard, is accomplished by comparing the cellular dimensions with those of the hexagonal reticules that correspond to determined densities. According to a second method, counting of the number of cellular elements is, instead, performed by using fixed or variable reticules.
While beneficial, neither method provides information as to the evolution of the cellular dimensions. Such information can be obtained, however, by identifying, in addition to the dimension of the average cellular area and its variability, the perimeters of the cells as well. This information is obtained through observation using an endothelium reflection microscope, which was first introduced in ophthalmologic practice in 1960 by David Maurice who, by modifying a metallography microscope, obtained photographic images of a rabbit's corneal endothelium. Using the same principles, a microscope was developed subsequently that was able to photograph the endothelium without contacting the eye.
Generally speaking, reflection microscopes of the non-contact type are derived from high magnification microscopes with normal slit lamps. These microscopes are based on the principle of visualization of a selected structure in relation to its ability to reflect an incident ray of light used for illumination. In the most commonly used technique (i.e., triangulation), the observation angle is about 45° , the microscope being oriented such that the bisecting axis of the angle of view is perpendicular to the plane tangential to the corneal surface.
Non-contact endothelium microscopy is particularly suitable for applications where contact with the cornea can be dangerous, such as immediately after surgery or in cases where the structure of the cornea is extremely fragile. By integrating the microscope with techniques of image analysis, the apparatus also provides a quantitative description of endothelium tissue, in the form of average cellular density and specific morphometric parameters.
In one conventional arrangement, a non-contact endothelium microscope apparatus is provided, which includes an optical unit with an illuminating system, for obliquely illuminating through a slit a surface portion of a patient's eyeball, and frontal eye observation, optical system, wherein an alignment-use indicator light for positional adjustment of the imaging optical axis is projected toward the patient's eye and the resulting reflected light is received and imaged by a TV camera. An enlarged-imaging optical system is also provided for enlarged observation, or enlarged photographing, of the subject surface portion on the TV camera based on slit illuminating light from which the eyeball surface has been illuminated.
In addition, a photo-detector is arranged so as to detect a position at which the enlarged-imaging optical system has been focused on the subject surface portion, via a reflected optical path other than that through which the enlarged image has been formed by the enlarged-imaging optical system. The optical unit is automatically moved, in response to the location of the indicator light displayed on a video monitor, both in a transverse direction and toward the eye, so that the location “chases” a specified position on the screen. In this manner, when the photo-detector detects focusing, the enlarged visual image of the subject portion of the cornea is photographed via the TV camera.
While this system has been found workable, placement of a focus detection, photo-detector along a supplementary reflected optical path, renders the apparatus complicated, and thus costly for providing and maintaining reliable results.